1. Field of the Invention
The present invention relates to a method which makes it possible to control optimally the temperature prevailing inside an oven when it is desired to heat a yarn in motion as, for example, during texturing operations, especially using false twisting, to which synthetic yarns are subjected.
2. Discussion of the Related Art
In the description which follows the method in accordance with the invention will be described more specifically for the control of an oven employed during an operation of texturing using false twisting of a polyamide 6.6 yarn, the type of oven controlled by virtue of the method according to the invention being an oven produced in accordance with the teachings of European Patent Application No. 524,111 (corresponding to US-A-5,193,293). It is obvious that this is not a limitation and that the invention can also be applied for controlling ovens of other types and different yarns (polyamide, polyester etc.), the internal temperature of the oven then being, understandably, adapted accordingly, but the inventive concept of the present application still being applied.
In all processes for the treatment of yarns in motion requiring a heat treatment the main problem which arises is that of fast heat transfer, it being necessary for the heat to enter the yarn uniformly throughout and similarly over its whole length. In fact, as is known, the temperature of treatment and its uniformity greatly influence the properties of the yarn produced.
Furthermore, it is well known that the heat treatment varies as a function of the material being treated, of the yarn count and of its speed of travel inside the oven. It is thus easily understood that the core of a fine yarn will be reached more quickly than that of one with a high count. Similarly, it is known that a yarn cannot be heated beyond some ceiling temperature, lest it be degraded.
Since the beginning of texturing, which now goes back nearly 50 years, very many solutions have been envisaged by people skilled in the art in order to produce ovens consuming as little energy as possible and permitting operation at increasingly high speeds (to give an idea, in the 1950s the production speeds were of the order of a few tens of meters per minute, whereas nowadays they reach a thousand meters per minute or even more). All these solutions consist in applying one of the three main principles of heat exchange, namely convection, radiation, conduction or their combination.
Nowadays, industrial texturing machines generally comprise ovens in which the heating of the yarn in motion is obtained by means of a heating liquid which, on vaporizing, transmits its heat to a body (plate) with which the fluid is directly in contact (see especially French Patent 2,619,128 or 2,473,565).
Since such a solution - which, from a technical standpoint, is satisfactory and results in yarns of good quality - entails the construction of increasingly longer ovens (nowadays they reach approximately two meters with processing speeds of the order of 800 m/min), people skilled in the art have concentrated on very old proposals, consisting in maintaining inside the oven a temperature which is clearly higher than the melting point of the substance of which the yarns are made, the latter traveling inside said ovens so that they reach an equilibrium temperature which makes it possible to carry out the treatment without, however, damaging or degrading the substance of which the yarn is made. Teachings along these lines emerge from Patents 1,204,634 and 1,117,718. However, operating in this manner, which can be defined by the expression "high-temperature treatment", presents numerous problems, especially with regard to the control, which must be carried out very precisely and must be adapted as a function of the yarns being treated. Furthermore, in such high-temperature ovens there arises the problem of keeping the yarn under specific constant tension without any vibration. These latter problems are solved satisfactorily in high-temperature ovens of the type described by the Applicant in its European Patent No. 524,111 (corresponding to US-A-5,193,293).
On the other hand, so far no proposal has been made for determining in a simple and rapid manner the precise temperature which must prevail inside such ovens to obtain an industrial output, it being absolutely necessary for the yarns to have the same characteristics not only from one machine to another, but also characteristics which are reproducible in time.
The present invention is aimed at solving such a problem.